Photocatalyzed reaction of indole in an aqueous suspension of titanium dioxide

Article abstract of DOI:10.1007/s11164-010-0122-9

The photocatalyzed oxidation of indole (1) in an aqueous suspension of titanium dioxide has been investigated and an attempt has been made to identify the products formed during the photooxidation process by gas chromatographic-mass spectrometric (GC-MS) analysis. Photolysis of an aqueous solution of indole (1) in the presence of titanium dioxide and oxygen led to the formation of 2,3-dihydroindole- 2-one (6) and 1H-indole-2,3-dione (7). A probable pathway for the formation of these products has been proposed. Springer Science+Business Media B.V. 2010.

Full text of DOI:10.1007/s11164-010-0122-9

Res Chem Intermed (2010) 36:121–125
DOI 10.1007/s11164-010-0122-9
Photocatalyzed reaction of indole in an aqueous
suspension of titanium dioxide
•
Mohammad Muneer Mohd Saquib
•
•
Mohd Qamar Detlef Bahnemann
Received: 31 July 2009 / Accepted: 3 December 2009 / Published online: 10 March 2010
ꢀ Springer Science+Business Media B.V. 2010
Abstract The photocatalyzed oxidation of indole (1) in an aqueous suspension of
titanium dioxide has been investigated and an attempt has been made to identify the
products formed during the photooxidation process by gas chromatographic–mass
spectrometric (GC–MS) analysis. Photolysis of an aqueous solution of indole (1) in
the presence of titanium dioxide and oxygen led to the formation of 2,3-dihydro-
indole-2-one (6) and 1H-indole-2,3-dione (7). A probable pathway for the formation
of these products has been proposed.
Keywords Photocatalytic oxidation Á Photocatalysis Á Indole Á Titanium dioxide Á
Semiconductor
Introduction
The photocatalyzed reaction of organic molecules in the presence of semiconductors
such as TiO2 has become a subject of serious study as it shows promise in becoming
a viable commercial technology for wastewater treatment [1–5]. This process bears
a close similarity to the charge separation developed in a photochemical cell,
containing semiconductor electrodes and the principles governing photoelectro-
chemistry apply equally well to reactions occurring on the surface of irradiated
M. Muneer (&) Á M. Saquib
Department of Chemistry, Aligarh Muslim University, Aligarh 202002, India
e-mail: readermuneer@yahoo.co.in
M. Qamar
Centre of Excellence in Nanotechnology (CENT), King Fahad University of Petroleum
and Minerals (KFUPM), Box 741, Dhahran, Saudi Arabia
D. Bahnemann
Institut fu¨r Technische Chemie, Universitat Hannover, Callinstr. 3, 30167 Hannover, Germany
¨
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M. Muneer et al.
particles [6]. On irradiation of the semiconductor at appropriate wavelengths an
electron is excited from the valence band to the conduction band, yielding an
electron/hole pair. The reaction of this electron–hole pair with a variety of electron
acceptors and donors, and the electron–hole recombination processes, have been
well studied [7, 8]. The formation of cation radicals of organic substrates, following
electron transfer to the excited semiconductor has been unambiguously character-
ized both from product analysis [9] and by spectroscopic studies [10]. The trapped
hole in TiO2 can lead to the formation of hydroxyl radicals, which can abstract a
hydrogen atom from weak C–H bonds and reacts with multiple bonds, including
reactions with aromatic systems, leading to the formation of free radicals. In the
presence of oxygen, these free radicals can be rapidly converted into unstable
peroxy radicals. Subsequent reactions of the peroxy radicals and other intermediates
formed can lead to the oxidation of organic materials dissolved in water. Of the two
possible pathways for the initial oxidation reactions namely, that involving direct
oxidation of organic substrate and the other proceeding through OH• radical-
mediated reactions, the current prevailing view favors the latter [11]. However, it is
also possible that the nature of the primary process may vary with the nature of the
molecule. Organic molecules, which can adhere strongly to the surface of the
semiconductors, for example, will be more susceptible to direct oxidation. Recent
pulse radiolysis studies seem to suggest that the surface-trapped hole and surface
bound OH• radicals may be indistinguishable [12].
The redox reactions of variety of organic molecules in acetonitrile have been
reported earlier [13–17] but very few reactions in aqueous systems have been
reported. For example, an aqueous suspension of 1,4-dihydrofuran in the presence
of zinc sulfide undergoes oxidation to give the coupling product [18–21].
An earlier study [22] has shown that the reaction of indole in the presence of
cadmium sulfide gives interesting products. In order to look into the nature of
products formed in the presence of titanium dioxide, we have studied the
photocatalyzed reaction of indole in an aqueous suspension of TiO2.
N
H
(1)
Experimental
Reagents
Reagent-grade indole was obtained from Merck and used as received without
further purification. Double-distilled water was employed for irradiation experi-
ments. P25 from Degussa was used as photocatalyst in this study. P25 consists of
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Photocatalyzed reaction of indole
123
75% anatase and 25% rutile with a specific BET-surface area of 50 m2 g-1 and a
primary particle size of 20 nm [23, 24].
Procedure
The model compound was dissolved in water and placed in an immersion well
photochemical reactor made of Pyrex glass (250 ml capacity). The required amount
of photocatalyst (Degussa P25, 1 g L-1) was added and irradiated using a 125-W
medium-pressure mercury lamp, with continuous stirring and purging with
molecular oxygen. The irradiated mixture was centrifuged and extracted with
chloroform, and the extract was subsequently dried over anhydrous sodium sulfate.
Removal of the solvent under reduces pressure gave a residual mass which was
analyzed by GC–MS. For GC–MS analysis a Shimadzu gas chromatograph and
mass spectrometer (GCMS-QP 5050) equipped with a 25 m CP SIL 19 CB
(d = 0.25 mm) capillary column. The column oven temperature was programmed
from 53 to 260 ꢁC at 10ꢁ min-1). Injection was in split mode (injection volume
1.0 lL), and helium was used as carrier gas.
Results and discussion
An aqueous solution of indole (1, 2 mM) was irradiated with a 125-W medium-
pressure mercury lamp in the presence of Degussa P25 (1 g L-1) and oxygen for
2 h. The catalyst was removed, the aqueous solution was extracted with chloroform,
the extract was dried over anhydrous sodium sulfate, and the solvent was removed
under reduced pressure to give a residual mass. GC–MS analysis of the residue
revealed the formation of 2,3-dihydroindole-2-one (6) appearing at retention time
(tR) 8.02 min. Prolonged irradiation of the model compound for 4 h (with work up
of the reaction mixture under analogous conditions) resulted in the formation of an
addition product appearing at retention time (tR) 10.55 min; this was identified as
1H-indole-2,3-dione (7). The products were identified by comparing their molecular
ions and mass fragmentation patterns (indicated below) with those reported in the
GC–MS library.
Compound 6: m/z, 133 (M?), 104, 89, 78, 63, 51, and 41
Compound 7: m/z, 147 (M?), 119, 104, 92, 76, 64, 50, 45, 41, and 38
Highly regioselective oxidation of the carbon near a heteroatom, for example
oxygen and nitrogen, has been accomplished on illuminated semiconductor
particles. For example, formation of benzoate esters by photooxidation of benzyl
ethers [17] and imides from lactams and N-acylamines in the presence of TiO2 [25].
The formation of products 6 and 7 from 1 involving electron-transfer reactions,
reaction with hydroxyl radicals and superoxide radical anion, formed in the
photocatalytic system, could be understood in terms of the pathways shown in
Scheme 1. The model compound 1 upon transfer of an electron can form the radical
anion 2, which may undergo addition of a hydroxyl radical followed by abstraction
of a proton to give 3. This compound, on oxidation under these reaction conditions,
can lead to the formation of the observed product 6. The abstraction of a hydrogen
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M. Muneer et al.
H
H
H
hυ
/TiO2
OH
H
H2O/O2
e
N
H
1
N
N
H
OH
H
3
2
[O]
H
OH
H
H
OH
/
O2
OH
H
O
N
O
N
N
O
H
H
H
6
5
4
[O]
O
O
N
H
7
Scheme 1
atom from 6, either by a hydroxyl radical or by a superoxide radical anion, can form
the radical species 5, which on subsequent addition of a hydroxyl radical followed
by oxidation can lead to the formation of the observed product 7.
Conclusion
Photocatalysis of indole in an aqueous suspension of titanium dioxide has been
found to result in oxidation at the 2 and 3 positions to yield two oxidized products.
Identification and characterization of the products formed during the photooxidation
process was a useful source of mechanistic information.
Acknowledgement Financial support by the Department of Science and Technology (DST),
Government of India, New Delhi, for a Young Scientist award to Dr M. Saquib (Project No. SR/FTP/
CS-25/2005), the Alexander von Humboldt Stiftung, Bonn, Germany, and by the Department of
Chemistry, Aligarh Muslim University, Aligarh, India is gratefully acknowledged.
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Photocatalyzed reaction of indole
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